1. Field of the Invention
The present invention relates to a method of controlling toner concentration in an electrophotographic developing apparatus, and more particularly to a method of controlling toner concentration in an electrophotographic developing apparatus that uses a two-component developer consisting of a carrier and a toner to allow the optimum mixing ratio of the carrier and the toner to be secured at all times.
2. Description of the Prior Art
Hitherto electrophotographic developing apparatuses that use a two-component developer consisting of a toner and a magnetic powder functioning as a carrier are known, and in such apparatuses, the image quality of the toner image (printed image) adhered to the latent electrostatic image on the photoconductor drum side is considerably influenced by the mixing ratio of the magnetic powder and the toner.
Further, in an electrophotographic developing apparatus using a two-component developer, even when the optimum mixing ratio is set originally, only the toner is consumed every time recording development is carried out (wherein the toner is stuck to the latent electrostatic image on the photoconductor drum), and the magnetic powder functioning as carrier remains in the developing container, so that the toner concentration in the developing container lowers, ultimately leading to an state incapable of development.
In order to obviate such a disadvantage, an apparatus for controlling toner concentration at a constant value is proposed that is based on the fact that toner concentration changes proportionally to the magnetic permeability, inductance, electric resistance, etc., and includes detecting means of detecting them in position in a developing container so that a prescribed amount of the toner is supplied suitably into the developer based on the detected output from said detecting means (Japanese Patent Publication No. 14896/1986).
However, in such an apparatus for controlling toner concentration, so-called overshoot, that is, excess supply of the toner occurs due to the delay of response of the detecting means, so that it becomes difficult to control the toner concentration in the optimum manner. Further, as the apparatus is made smaller as in recent years, the toner supply rate becomes high relatively to the volume of the developer in the developing container, so that in practice the above disadvantage has become too conspicuous to be ignored.
In order to obviate the disadvantage, a control apparatus is developed wherein the detecting operation of said detecting means and the supplying operation are separated, and every time the lowering of the toner concentration to or below a certain level is detected by said detecting means, a prescribed amount of the toner is supplied batch-wise. In this apparatus, unrequired overshoot due to the delay of the response of the detecting means can be obviated, but accurate control cannot be effected because the control is so-called zone control wherein every time the toner concentration lowers to a certain level, a prescribed amount of the toner is supplied.
The toner concentration of the developer tends to fluctuate due, for example, to the stirring action by a stirring member and the timing of the start of the developing operation, and since any of the above prior techniques grasps the toner concentration not as an average value within a certain period but as a momentary value, in some cases, the detecting means judges erroneously the toner concentration to be a control standard level or below even in the event that the toner concentration is normal, and as a result the toner is supplied by the wrong control, resulting in an excess toner concentration.
To obviate such a disadvantage, a technique capable of correcting the fluctuation of the output of a detecting means immediately after the start of the operation of a developing section is proposed wherein a correction signal corresponding to the difference between the output signals of the detecting means at the time when the operation of the developing section is stopped and at the time when the operation is resumed is produced, the thus produced correction signal is added to the output signals of the detecting means at the time when the operation of the developing section is started and at the time when the next operation is started thereby carrying out the correction, and the correction signal is attenuated by a predetermined time constant (Japanese Patent Laid-Open (Kokai) No. 24641/1979).
Although the prior technique can deal with the fluctuation of toner concentration due to the timing of the start of the operation of the development, it cannot deal with the fluctuation of toner concentration due to the stirring operation during the developing operation.
In order to prevent a detection error due to the momentary grasp of the toner concentration, a technique is disclosed wherein a concentration detecting circuit that compares the detection signal (voltage) of the toner concentration with a standard concentration level (voltage) for example as shown in FIG. 5 to produce a level signal "Hi" when the detection signal of the toner concentration is at or below the standard concentration level, and a level signal "Lo" when the detection signal of the toner concentration is at or above the standard concentration level, a pulse initiator for producing a clock pulse signal Ck corresponding to a prescribed standard clock, and an AND circuit that takes "AND" of said clock pulse signal Ck and the level signal "Hi" to produce its logical product signal Rs are includes, a toner supply signal Ts1 is produced synchronously with said logical product signal Rs to effect a prescribed supply operation, when said logical product signal Rs has reached a prescribed number of counts or over, in other words, when the prescribed number of the toner supplies has recurred, that is automatically detected thereby thinning said logical product signals Rs and switching to time supply that will produce a toner supply signal Ts2 at every prescribed interval, and after said time supply is effected for a prescribed period, said toner supply signal Ts1 is produced again to repeat the above operation (Japanese Patent Publication No. 42873/1981).
In this prior technique, when the toner supply is repeated prescribed times, since the technique uses such a toner supply operation that the logical product pulse is changed to the thinned pulse (time supply), excessive supply of the toner can be prevented, but if development that consumes a large amount of the toner as in solid black development is effected, toner supply cannot follow quickly the consumption, as a result the image quality becomes thin disadvantageously, and since the technique uses pulse (intermittent) supply control based on the logical product signal Rs, if the toner concentration has decreased sharply, it is quite difficult to recover the toner concentration quickly.